1. Field of the Invention
The present invention relates to a radar unit and, more particularly, to a radar angle correction method for correcting an error in a radar angle caused by the front cover of a radar antenna.
2. Description of the Related Art
As far as radar units to be installed outdoor or mounted in a vehicle are concerned, if the surface of an antenna is exposed to the outside of the radar unit, there is the fear that dust and dirt may adhere to the surface and cause a malfunction of the antenna. For this reason, in general, the antenna is stored in a predetermined container, and the front side of the container oriented in a radio-wave transmitting/receiving direction is shielded with a resin cover that does not cause a drop in the power of radio waves or a distortion of a beam.
For example, an on-vehicle millimeter-wave radar unit (FM-CW radar unit etc.) transmits radio waves to a target so as to measure a distance from the target, a relative speed of the target, and an angle thereof. Herein, a carbonic resin that is superior in mechanical strength and exhibits a small attenuation ratio for radio waves is adopted as a material for the cover.
Moreover, in general, the radar unit mechanically or electronically sweeps a transmitted beam in a predetermined direction, and fits the distribution of received beam power into a quadratic curve so as to calculate the angle of the target.
Product inspection is performed on a finished radar unit including a front cover. A correction value, that is determined based on the result of the inspection and used to minimize a difference in a characteristic of a product from others, is stored in a nonvolatile memory incorporated in the radar unit. The radar unit is then delivered (refer to, for example, Patent Documents 1 and 2).
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-22833 (see FIG. 5)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-69355 (see FIG. 2)
Incidentally, the carbonic resin cover is hardly susceptible to radio waves. Nevertheless, the susceptibility varies depending on an angle formed between the cover and the surface of the antenna. Other materials to be made into the antenna cover exhibit similar properties.
The on-vehicle millimeter-wave radar unit and other radar units that move (rotate) the antenna in a predetermined direction and fit a distribution of received power into a theoretical quadratic curve so as to detect the angle of a target, have a drawback that the angle of the target cannot be accurately calculated because a degree of attenuation of a received power varies depending on the angle between the front cover and antenna surface.
The above drawback becomes obvious especially when a radar unit is inspected in order to adjust the axis thereof. When the axis of a radar unit is adjusted on the premises of a narrow factory, the radar unit that is an object of inspection cannot be arranged far from a reference target used to adjust the axis. Therefore, a level variation factor attributable to the angle of the front cover through which a signal passes, is superposed on an angle signal whose received level hardly differs from the original signal level because of the distance of the target. This makes it harder to accurately adjust the axis of the radar unit.